Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation. Optoelectronic devices that make use of organic semiconductor materials are becoming more desirable because of their potential for cost advantage over inorganic semiconductor materials and certain beneficial inherent properties organic materials, such as their flexibility.
Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic (“PV”) devices, are a type of photosensitive optoelectronic devices that are specifically used to generate electrical power. An organic photosensitive device comprises at least one photoactive region in which light is absorbed to form an exciton, which may subsequently dissociate into an electron and a hole. The photoactive region will typically comprise a donor-acceptor heterojunction, and is a portion of a photosensitive device that absorbs electromagnetic radiation to generate excitons that may dissociate in order to generate an electrical current. The donor-acceptor heterojunction can be a planar heterojunction, bulk heterojunction, or hybridized mixed-planar heterojunction. A hybridized mixed-planar heterojunction comprises a first organic layer comprising a mixture of an organic acceptor material and an organic donor material; and a second organic layer comprising an unmixed layer of the organic acceptor material or the organic donor material of the first organic layer. Such hybridized mixed-planar heterojunction is described in United States patent application Publication No. 2005/0224113 of Xue, et al., published on Oct. 13, 2005, the contents of which are incorporated herein by reference in its entirety.
An organic photosensitive optoelectronic device may also comprise transparent charge transfer layers, electrodes, or charge recombination zones. A charge transfer layer may be organic or inorganic, and may or may not be photoconductively active. A charge transfer layer is similar to an electrode, but does not have an electrical connection external to the device and only delivers charge carriers from one subsection of an optoelectronic device to the adjacent subsection. A charge recombination zone is similar to a charge transfer layer, but allows for the recombination of electrons and holes between adjacent subsections of an optoelectronic device. Charge recombination zones are described, for example, in U.S. Pat. No. 6,657,378 to Forrest et al.; Published U.S. Patent Application 2006-0032529 A1, entitled “Organic Photosensitive Devices” by Rand et al., published Feb. 16, 2006; and Published U.S. Patent Application 2006-0027802 A1, entitled “Stacked Organic Photosensitive Devices” by Forrest et al., published Feb. 9, 2006; each incorporated herein by reference for its disclosure of recombination zone materials and structures. A charge recombination zone may or may not include a transparent matrix layer in which the recombination centers are embedded. A charge transfer layer, electrode, or charge recombination zone may serve as a cathode and/or an anode of subsections of the optoelectronic device. An electrode or charge transfer layer may serve as a Schottky contact.
For additional background explanation and description of the state of the art for organic photosensitive devices, including their general construction, characteristics, materials, and features, U.S. Pat. Nos. 6,972,431, 6,657,378 and 6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic et al., are incorporated herein by reference in their entireties.
In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (“HOMO”) and Lowest Unoccupied Molecular Orbital (“LUMO”) energy levels of two contacting but different organic materials. If the HOMO and LUMO energy levels of one material in contact with another are lower, then that material is an acceptor. If the HOMO and LUMO energy levels of one material in contact with another are higher, then that material is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material.
As used herein, a first HOMO or LUMO energy level is “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level and the first HOMO or LUMO energy level is “lower than” a second HOMO or LUMO energy level if the first energy level is further away from the vacuum energy level. A higher HOMO energy level corresponds to an ionization potential having a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electron affinity having a smaller absolute energy relative to vacuum level. On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In the context of organic photosensitive devices, a material that conducts preferentially by electrons due to high electron mobility may be referred to as an electron transport material. A material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport material. A layer that conducts preferentially by electrons, due to mobility and/or position in the device, may be referred to as an electron transport layer. A layer that conducts preferentially by holes, due to mobility and/or position in the device, may be referred to as a hole transport layer. Preferably, but not necessarily, an acceptor material is an electron transport material and a donor material is a hole transport material.
As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substitute does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule.” In general, a small molecule has a defined chemical formula with a molecular weight that is the same from molecule to molecule, whereas a polymer has a defined chemical formula with a molecular weight that may vary from molecule to molecule. As used herein, “organic” includes metal complexes of hydrocarbyl and heteroatom-substituted hydrocarbyl ligands.
An example of organic optoelectronic devices that produce electromagnetic radiation electronically include organic light emitting devices (OLEDs). OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, the disclosures of which are incorporated herein by reference in their entireties.
OLED devices are often configured to emit light through at least one of the electrodes, and one or more transparent electrodes may be useful in an organic opto-electronic devices. For example, a transparent electrode material, such as indium tin oxide (ITO), may be used as the bottom electrode. A transparent top electrode, such as disclosed in U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated herein by reference in their entireties, may also be used. For a device intended to emit light only through the bottom electrode, the top electrode does not need to be transparent, and may include a thick and reflective metal layer having a high electrical conductivity. Similarly, for a device intended to emit light only through the top electrode, the bottom electrode may be opaque and/or reflective. This is because, where an electrode does not need to be transparent, using a thicker layer may provide better conductivity, and using a reflective electrode may increase the amount of light emitted through the other electrode, by reflecting light back towards the transparent electrode. Fully transparent devices may also be fabricated, where both electrodes are transparent.
In many color display applications, three OLEDs, each emitting light of one of the three primary colors, blue, green and red, are arranged in a stack, thereby forming a color pixel from which any color can be emitted. Examples of such stacked OLED (“SOLED”) structures can be found described in PCT International Application WO 96/19792 and U.S. Pat. No. 6,917,280, the disclosures of which are incorporated herein by reference in their entireties.
In such a stacked structure, a pair of electrode layers are provided, one at the bottom and another at the top of the SOLED stack. In one variation of SOLEDs, an intermediate electrode layer that is externally connected can be provided between each of the OLED units in the stack. In other variations of SOLEDs, a charge generating layer (“CGL”) that injects charge carriers but without direct external electrical connection is provided between each of the OLED units in the stack.
As used herein, “top” means furthest away from the optoelectronic device's substrate, while “bottom” means closest to the substrate. For example, for a device having two electrodes, the bottom electrode is the electrode closest to the substrate, and is generally the first electrode fabricated. The bottom electrode has two surfaces, a bottom surface closest to the substrate, and a top surface further away from the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate but not necessarily in physical contact with the second layer. There may be one or more other layers between the first and second layers, unless it is specified that the first layer is “in physical contact with” the second layer. For example, a cathode may be described as being “disposed over” an anode, even though there are various layers in between.